1. Field of the Invention
The present invention relates to a semiconductor memory device, more particularly to a semiconductor memory device having a redundancy circuit.
2. Description of the Related Art
Some types of semiconductor memory devices consist of a combination of a random-access memory (RAM) and a shift register group. This type of semiconductor memory device is, in general, used as an image memory. The shift register group is attached to the RAM for high-speed read-out of the data stored therein. The RAM is, for example, provided between a central processing unit (CPU) and a cathode-ray tube display (CRT). The data stored in the RAM is read out in parallel and input to the shift registers in accordance with control of a system. The read-out data is output serially from the shift registers to the CRT.
This type of semiconductor memory device thus enables both low-speed random access by the CPU and high-speed serial data transmission between the RAM and the CRT. In this regard, however, there has been no disclosure of a circuit giving a redundancy function to the RAM. In other words, while there is a well-known RAM having a redundancy circuit, there is no RAM having both a high-speed serial data output function and a redundancy function. Yet, a redundancy circuit is essential to raise the yield in production of a RAM.